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From: George Huxtable (no email)
Date: Tue Mar 07 2006 - 19:03:33 EST
Frank Reed wrote-
| George H wrote:
| "Ptolemy, in the early pages of his "Almagest", in about 200 AD, considers
| the arguments for and against motion of the Earth, and decides that indeed the
| Earth is stationary at the centre of the Universe. I doubt if many of us
| would have argued otherwise, if we had found ourselves in the same situation, in
| the same state of knowledge."
|
| I agree with that completely, and I would even go a little farther. If we
| were placed back in time 1800 years with our modern sense of scientific
| methodology and principles intact, we would still reach the same conclusion. The
| most damning evidence against the motion of the Earth is the apparent lack of
| stellar parallax. The reply from advocates of a moving Earth --"well... maybe
| the stars are really far away..."-- would immediately strike us an ad hoc
| assumption designed to avoid facing up to observational evidence that clearly
| rules out the theoretical model. That the ad hoc assumption turns out to be
| entirely correct is one of those things that makes the history of science
| interesting...
==============
Indeed, the lack of stellar parallax was an important, even a clinching argument agains the
implausible notion that the Earth might be making a circuit around the Sun. It's nice to agree so
well with Frank.
Perhaps it might be of interest to set out the steps in that argument, as it appeared to the Greeks.
By "the Greeks", I'm really referring to that late, great authority on these matters, Ptolemy, in
about 200 AD, but even in the time of Hipparchus, 3 centuries earlier, such matters were rather well
understood.
First, there was no doubt that the Earth was a sphere, and its radius was quite precisely known, by
the differing altitudes of the noon Sun from different latitudes, a known distance apart.
The distance of the Moon was also reasonably well known. There were a number of ways of getting a
handle on that. One was the shape of the Earth's shadow, cast on the Moon at the time of a lunar
eclipse, which gave an idea of the relative sizes of Earth and Moon. Another was the way that the
totality of a solar eclipse varied with position of the observer on the Earth's surface. The Moon's
distance is actually about 60 Earth radii, and the Greeks had that nearly right.
It was clear that the Sun was many times further away from the Earth than was the Moon, from
measurements made when the Moon's disc was exactly half-illuminated by the Sun. Then the angle
between Moon and Sun should give their relative distances by triangulation, but no difference from
an angle of 90 degrees could be distinguished. So all that could be deduced was that the Sun was at
least many times (more than 10, say) further than the Moon, whereas in fact it is nearly 400 x the
Moon's distance.
Even so, that meant that if the Earth was going round the Sun, the radius of its journey would be at
least something like 1000 Earth radii (in fact, it was 24 x greater still). In that case, they would
argue, you would expect to see some apparent motion of stars in the sky, as the viewpoint from the
Earth changed by such an immense amount over the year. But no such relative motion (parallax) could
be detected, between stars in one direction and stars in another. If the Earth really was moving
round the Sun, the stars would have to be inconceivably distant, for there to be no discernable
parallax. We now know, of course, that that was indeed the case. To the Greeks, however, it was more
logical to presume that the Earth stayed put at the centre of the universe. Can we blame them for
rejecting the other possibility? I think not.
George.
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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